a) Field of the Invention
The present invention relates to an extrusion method for obtaining a phenolic resin pipe, an extruder in effecting said method, and an extrudate obtained from said method and extruder.
b) Description the Prior Art
As methods for molding a thermosetting resin, there are known compression molding, transfer molding, injection molding and extrusion. Various apparatuses suitable for respective methods are in use.
With respect to the extrusion as one of the methods for molding a thermosetting resin, plunger extrusion is generally used. For example, Japanese Patent Application Laid-Open No. 83155/1973 and Plastics Vol. 25, No. 3, p. 47 describe the production by plunger extrusion, of long shaped articles of simple configuration such as round bar, pipe and the like.
Plunger extrusion, however, makes it difficult to produce an extrudate of homogeneous quality and gives low productivity because it employs a high extrusion pressure in the mold cavity of the plunger extruder and is conducted intermittently.
From the above reasons, extrusion using a so-called screw type equipment is disclosed in, for example, Japanese Patent Application Laid-Open No. 23661/1979 or Japanese Patent Application Laid-Open No. 18949/1974. Such extrusion uses an extruder wherein a thermosetting resin is kneaded and melted in an extrusion unit and the molten resin is extruded from the unit and introduced into a die unit through an adapter or the like in a time as short as possible to prevent the progress of the thermosetting reaction and shaped into a final form.
With such an extruder, however, continuous and stable extrusion has been difficult because the resin has a complex flow pattern and, as a result, slight fluctuations in temperature and/or rapid progress cause rapid progress of thermosetting reaction, or appearance of flow stagnation gives rise to localized curing reaction.
That is, in the extrusion disclosed in, for example, Japanese Patent Application Laid-Open No. 23661/1979, the resin heated and melt in a cylinder is introduced into a die through an adapter and shaped into a final form. In this procedure, the resin is squeezed and then expanded around a mandrel fixed by a spider; thus, the resin makes complex flow. Consequently, stagnation of flow takes place easily, giving rise to localized curing reaction, or slight fluctuation in pressure and/or temperature causes rapid appearance of curing reaction. In order to extrude a resin while overcoming the resistance caused by complex flow and preventing the resulting flow stagnation, a very large extrusion pressure is required and a special extruder capable of generating such a pressure is needed.
Further with the above extruder, it has been unable to prevent quality problems such as (a) spider marks caused by a spider used for supporting a mandrel and (b) welds generated owing to the presence of the spider.
In order to solve these problems, extrusion by a screw extruder employing a screw having a smooth zone at the front end is proposed in, for example, U.S. Pat. No. 4,797,242, and is currently used for production of phenolic resin pipe. In this extruder, however, the smooth zone at the front end of the screw corresponds to the inside diameter of a pipe to be produced and accordingly, in view of the wall thickness of the pipe, it is impossible to produce a pipe having an outside diameter smaller than 20 mm because of the restriction of the mechanical strength of the screw. Further, since the screw and the cylinder also affects the operation of the die, production of a pipe of new size requires designing a new screw and a new cylinder, providing an economical disadvantage particularly in the production of a variety of products in small amounts.
U.K. Patent No. 2089717 discloses an extruder which corresponds to the extruder of Japanese Patent Application Laid-Open No. 23661/1979 minus the adapter and the spider and which comprises a screw, a cylinder, a die and a mandrel, wherein the mandrel is provided on the extension of the screw axis. This extruder was proposed for production of thermoplastic resin pipe. This extruder relates to an extrusion technique for obtaining a pipe of balanced strength by utilizing the property of molten thermoplastic polymer that their molecules, when flowing through a small path at a high speed, are orientated to the flow direction and by orienting the polymer molecules to different directions at the inner and outer layers of the pipe. In order to achieve the orientation in different directions, there was proposed the use of an extruder having a torpedo on the extension of a screw. This extruder was proposed for application to thermoplastic resins, as mentioned above. When the extruder was applied to extrusion of phenolic resin, i.e. thermosetting resin, extrusion became impossible in a short time after the start of pipe delivery from the die, making it impossible to obtain a satisfactory shaped article. The progress of partial curing at the die inlet was presumed to be a reason for inviting failure in extrusion.
Thus, no fully satisfactory extrusion technique has been developed for obtaining thermosetting resin shaped articles.